A surprising byproduct of wildfires: Contaminated drinking water (2024)

Over the weekend, the Park Fire grew to more than 360,000 acres, prompting evacuation orders and warnings around Chico, Calif. in Butte, Plumas, Shasta and Tehama counties. In the days ahead, Cal Fire will seek to contain the blaze to reduce harm to people, structures and the environment. However, months from now when the rains come and the fires are extinguished, a hidden threat could put communities at risk once again.

When the mayor of Las Vegas, N.M., issued a warning in 2022 to its 13,000 residents, it wasn’t over a fire — they had recently lived through the state’s largest wildfire in its history: Calf Canyon/Hermits Peak. The dire warning was that the city had 30 days of clean water left. The 2022 monsoon rains covered the Gallinas watershed, where cleared trees from the Santa Fe National Forest and ash-covered grounds made for flash-flood conditions. The storms introduced massive amounts of carbon from burned trees and plant life into the streams and reservoirs. Water treatment couldn’t keep up, making their stores undrinkable.

Many drinking water sources at risk from wildfires

Around 60 to 65 percent of the United States’ drinking water comes from forested areas. As fires burn in these areas, they increase the risk of cancer-causing and toxic substances entering water supplies. An estimated 53.3 million U.S. residents who live in areas with significant wildfire risk may face damaged drinking water infrastructure from those flames.

Map of at-risk watersheds in the US Western States

The new megafire era

Randy Dahlgren, a professor emeritus at the University of California at Davis whose research focused on wildfires and watersheds in California, says that fires’ impact on clean water boils down to the size, intensity and severity of the fire.

The 2022 Calf Canyon/Hermits Peak wildfire is among the largest in recorded history in the United States. The fire perimeter stretched across 340,000 acres, with high burn severity in most areas. Thousands were forced to evacuate during the course of the months-long blaze. These megafires — fires greater than 100,000 acres — of the 21st century are increasingly common due in large part to the persistently drier and hotter conditions of forested areas in a warming climate.

“I would project that both the size and the severity of wildfires are going to increase,” says Dahlgren. Post-megafire fallout — because of their scale and intensity — is linked to poorer water quality during the following rainy seasons, Dahlgren adds.

Megafires burn land at higher temperatures across wider areas than standard wildfires, putting watersheds across the United States at greater risk. Sheila Murphy, a research hydrologist at the U.S. Geological Survey working on the effects of wildfires on water quality, says burned areas fundamentally alter a watershed’s hydrology. As wildfires burn hotter and consume more trees and structures, water quality will continue to worsen, research suggests.

When watersheds burn, the threat starts in the forests, continues to water treatment plants, and can expand to communities and households. To meet these risks, it will take a coalition of informed community members, scientists and city officials to work toward solutions to protect clean water supplies.

See how fires alter these systems and introduce contaminants.

A table of contents for the next section of the story showing that the piece will cover how the fires impact healthy forests, communities and households.

Scientists are only beginning to study the effects megafires pose on local ecology.

Newsha Ajami, the chief strategic development officer for research in the Earth and Environmental Sciences Area at Lawrence Berkeley National Labs, studies the multifaceted ways fires impact forest management, water utilities and communities. Ajami says on the state of the science, “We’re destabilizing [water] systems and we don’t even know in what way and how.”

A first set of guidelines

In 2019, a team of enterprising hydrologist-planners realized that there needs to be a handbook for this new reality of wildfire impacts on communities. That fall, volunteers from more than a dozen municipalities wrote the Post-Fire Recovery Playbook, a first-of-its-kind 12-page concise document for water municipalities, land and forest management, and governments to rebuild within 30 days of a fire.

Ecologists and researchers working with post-fire effects on water juggle hyperlocal environmental needs with the needs of neighboring communities and the resources of their governments. The authors of the recovery playbook found that measures need to be in place for communities of any size to handle the aftermath of nearby fires. The guide highlights the “gap in guidance in terms of navigating the complexities surrounding post-fire rehabilitation.”

Insuring right and rebuilding smarter

Ajami sees a critical role for insurance companies implementing smart, resilient practices for communities left to cinders. Similar to how health insurance is trying to focus on preventive care to reduce the cost of treating disease, Ajami hopes “at some point we will have a preventive insurance model that would invest in actions people can take from being impacted.”

“That’s something we’re starting to touch on now in terms of research,” says Andrew Whelton, a professor at Purdue University working in civil, environmental and ecological engineering. Whelton says that “the insurance industry needs to understand water contamination, water safety, what the alternatives are and how much they cost.” The demands are in understanding costs so they can effectively get ahead of these disasters. “Simply put, insurance companies haven’t anticipated these costs and they haven’t anticipated the cost at the scale they are being hit at.”

When the 2020 CZU Lightning Complex fire climbed the side of a mountain into a housing development in Boulder Creek, Calif., toxic contamination didn’t spread from the homes back into the water system. That was because the San Lorenzo Valley district utility made sure the valves in that network prevented backflow. Preventive steps like that, Whelton says, go a long way in improving fire resiliency and keeping community drinking water safe. He would know, too: Whelton has become the go-to expert flying out to nearly every megafire that’s burned down towns since 2017, working with communities and municipalities on testing water.

Another prevention area is setting up proper lines of communication. Three days after the Maui fires and evacuations began in 2023, the Maui County Department of Water Supply issued an “Unsafe Water Advisory.” All Maui households surveyed in a recent study by Whelton and several other researchers “expressed concerns or confusion about drinking water safety” two weeks after the fire.

Communication challenges, like rapidly getting information to water customers after a fire, are what led Whelton and his colleagues to co-author the Wildfire Response Guide for Environmental Public Health Professionals for the National Environmental Health Association. Guides that reduce friction between local and state governments and health professionals assist in risk-monitoring communities post-fire. For protecting water, the guide explicitly highlights where water can be compromised, offering damage-assessment guidance, community messaging on safety risks, and what kinds of testing to prioritize.

Further downstream, researchers contend with how to build homes in fire-prone areas to make them more resilient and leach less hazardous waste in the event of a disaster.

Erica Fischer, a civil engineer at Oregon State University who spent her PhD studying fire impacts on buildings, saw firsthand the devastation wrought by wildfires in communities like Paradise, Calif., and Louisville, Colo. She’s witnessed conversations move away from just forest management to home-hardening and resiliency-building. Legislation like Oregon’s Senate Bill 762 — which put $220 million into wildfire preparedness of buildings, landscapes and emergency response — Fischer notes, not only puts resources into mapping wildfire risk after the 2020 Oregon fires, but also provides financial assistance for the socially vulnerable in the rural communities at risk.

Other legislation is also being advanced. The EMBER act, a bipartisan bill introduced in June by Sens. Mitt Romney (R-Utah) and Mark Kelly (D-Ariz.), aims to modernize national wildfire policies, including by updating recovery guidance on drinking water toxicity resulting from wildfires.

With practices like this in place across wildfire risk areas, the likelihood of a watershed weathering subsequent high rains improves. Dahlgren hits on the adage: “Dilution is the solution to pollution.” If there are backups in place, diluting at least the DOM water can provide safe drinking water downstream.

Planning for a future with wildfires

Meanwhile in Boulder, Colo., they are divining the future of fires. Kate Dunlap, a Post-Fire Recovery Playbook co-author and manager of Boulder’s Drinking Water Quality program, applies machine-learning algorithms to identify where to place resources for disaster preparedness. One model simulated 10,000 burns in the watershed according to local topography, vegetation and geology. The results allow Dunlap’s team to understand how much risk there is, where disasters can come from and how much it’ll cost to prevent or treat them.

These forecasts help cities predict how much sediment could enter reservoirs. Knowing where hazardous debris might come from allows the government to prioritize stabilizing at-risk forested areas while keeping costs low.

The science is trying to keep pace with an ever longer and more frequent fire season. Researchers consulted for this story are studying fires from 2021 and 2022, as well as conditions generated by these newer, larger fires that burn in regions geographically, vegetation-wise and community-wise different from where previous fires burned. Additional research is critical to understand how this global risk impacts communities at a local level.

Dunlap sees a lot more urgency here in her field. In eight years on the job, it’s only since 2020 that she’s witnessed increased funding sources at the state and federal level for forest health projects. The general awareness shift she’s seen is that “wildfires are real and we’re not really experiencing these sort of natural fires anymore. They’re very severe.”

The community of Las Vegas, N.M., is preparing for the next big fire by strengthening their water supplies along the Gallinas River, both upstream by the wildfire-prone regions and downstream at the household level. Johanna Blake, a U.S. Geological Survey researcher at the New Mexico Water Science Center, sent The Washington Post photos of the rocky barriers — gabions — that now stretch along the river bed. After the rainy season in 2022, she could smell the collected ash that got stopped before entering the river there.

About the story

The wildfire risk to watersheds were calculated from the U.S. Forest Service’s Forest to Faucet data. A watershed was categorized as “Great risk” if 70% or more of its total acres had a “high or very high wildfire hazard potential.”

The number of surface drinking water consumers for each watershed is based on late 2018/early 2019 population estimates.

Only watersheds that were entirely contained within state boundaries were included in the calculation of residents at risk of wildfire water contamination in Washington and California. Watersheds that intersected with state lines were not included in the calculation to avoid double counting.

Janice Kai Chen contributed to this report.